Intestinal Parasites in the Horse: Some New Thoughts on Damage Control.
(Everything You Ever Wanted to Know About Worms)

Ron Friedman M.S., D.V.M., D.A.C.T.

Intestinal "worms"! The thought is enough to turn some peoples stomachs (no pun intended). Other’s think they aren’t a real problem and the regular use of dewormers is only a plot by drug companies to sell their product. An argument often put forth is "wild horses don’t have parasite problems"... The fact is worms are a widespread reality in the lives of horses and will continue to be so. This persistent problem is complex and involves the horse, its pasture, and parasites. In this article we’ll try to illustrate the total picture of worms so as to fully understand the "truth and consequences" of intestinal parasites or worms.

Two questions frequently asked include: are worms a problem needing control? And, haven’t horses and parasites existed together for millions of years without intervention from man? The answers are revealed when we look at how wild horses lived and how horses are housed today.

Wild Horses and Parasites

Parasites serve a very important population control function in the natural ecosystem of the horse. "In the wild" when horse populations get too large for the ecosystem to support, worm populations increase and horses die. In the wild horse several very important factors serve to control the damage worms cause. Wild horses graze on large tracts of rangeland, and historically these rangelands were dry arid regions with sparse vegetation. Horses would graze on vegetation and roam as much as 25 miles daily. While grazing, horses which were infected shed worm eggs along with their feces onto the ground. Some parasite eggs can directly cause infection when eaten by the horse, other parasites need to hatch and develop through several larval stages (like tadpoles into frogs, or caterpillars into butterflies) before they can enter the horse. By the time these larvae were infective the horses had usually roamed far away. The climate in which horses "naturally" live is also very hard on worms. Many parasites don’t do well in hot dry areas, in contrast the climates where most people live, temperate moist environments, are beneficial for worm development.

So to answer the previous questions: yes parasites can be a problem and their numbers must be monitored and controlled. And, yes horses and parasites have existed together for millions of years without the help of humans BUT when we started changing the way horses were housed serious problems arose.

Importance of Parasite Control

The importance of good parasite control measures can sometimes be hard to convey. It is often difficult to detect early parasite damage and even more difficult to establish a link to the parasites themselves. The problems don’t clearly show how parasites originate. It is important to remember that if a horse looks wormy the problem has gone too far. A pamphlet published by the American Association of Equine Practitioners on parasite control explains the importance of this topic well: "In terms of management priorities, establishing an effective parasite control program is probably second only to supplying the horse with clean plentiful water and high quality feed. It's that important!"

Some parasites can be detected by observing the adult in manure, others by microscopic examination of feces, and others remain undetectable by these methods even though they are doing their damage. As many of you have read other articles on the variety of worms and their lifecycles, this article will only cover those topics superficially. We think it more important that people understand the total picture about intestinal parasites and hope this article gives you some new ideas.

At this point we’d like to clear up a few common misconceptions: many horse-owners mistakenly believe that deworming results in the elimination of worms. Under certain circumstances this may be true for a variable and short period of time (mature horse living alone, on plentiful pasture). Mostly though, stabling in public barns, or any situation in which horses are grouped together usually results in a more persistent infestation. The whole issue is not of eliminating intestinal parasites, but of decreasing their numbers. Another common misconception is that the parasite problem is under control if the horses don't look "wormy" (rough hair coat, stunted growth, tail rubbing, emaciation, etc.). This is not true. Parasites injure horses slowly in a variety of ways. Some of the damage done by worms is severe others not. Once these symptoms are seen, significant internal damage has occurred, some of which are irreversible. A good parasite control program instituted prior to the appearance of symptoms is effective, efficient and far better for the health of the horse.

Parasitism of horses is best controlled when thought of and treated as a herd or farm infection, not an individual horse problem. The most successful parasite control programs include regular deworming with appropriate dewormers, manure disposal, and semi-annual fecal exams to monitor the effectiveness of the controls. One very effective control measure has unfortunately lost its popularity (because of the effort involved) - namely manure pickup and disposal from the pasture; we'll discuss this more when we review the control of parasites.

If we were to consider the different lifecycles, and the damage done by most intestinal parasites the details would easily fill this entire magazine. To very briefly summarize: although different parasites have different life cycles the basic pattern involves eggs hatching, developing into larvae, migrating throughout the body (often this migration is the worst damage), and maturing into adults which lay eggs.

Adults lay from hundreds of thousands to millions of eggs per day!! Adults are often found in groups of hundreds or thousands. Horses eat grass, grain, or hay contaminated with manure and become infected with the eggs or larvae. Some parasites physically invade the horse via the mouth or skin but the fecal-oral route is most common. This information is important to remember when we review control measures below.

Common Intestinal Parasites

The types of common intestinal parasites which are of the most concern are: Large Strongyles (bloodworms), Small Strongyles, Bots, Pinworms, and Ascarids (roundworms). Other less common worms are Threadworms, Stomach worms (summer sores), and Lungworms but these usually cause less severe problems.

Large Strongyles are the most common equine intestinal parasite and will be discussed in more detail later. A very brief description of the other intestinal parasites follows.

Small strongyles attach to the wall of the small intestine and can cause diarrhea, loss of appetite, and colic. They can be frequent causes of illness and are often seen with other parasites. Small strongyles have also developed a unique survival adaptation, the ability to encyst themselves. This ability involves the parasite migrating through the tissues of the horse where under certain situations they stop migrating and become encysted by the horse. This ability to encyst themselves ensures survival of the species. The way it works is that during its migration the small strongyle larvae receive a signal from adults within the intestine of the horse. This signal tells the migrating larvae not to enter the intestine, there are enough adults laying eggs. When the larvae stop migrating the body attacks them by surrounding them with scar tissue and they become encysted. The encysted small strongyle will remain in this stage as long as there are adults laying eggs in the intestines. When we kill these adults by deworming, the encysted worm finds out by an unknown feedback loop and some will resume their migration towards the horse’s intestine and start laying eggs. . Small Strongyles have also shown the most resistance to the Benzimidazole type dewormers and more will be mentioned about this later. With the widespread use of Ivermectin (which is ineffective on encysted Small Strongyles- but very effective on most other worms) and increased resistance to dewormers, this worm has become the most common internal parasite in some areas of the country.

Bots are very persistent and in low numbers are not a problem. Some horses however, are particularly bothered by the adults (large flies) and have been known to go to extreme measures to avoid them (running through fences). The larval form live in the stomach and migration can cause ulcers in the mouth. These ulcers result in pain and can lead to all sorts of problems associated with decreased appetites (e.g. colic).

Pinworm's damage is due to the intense itching and scratching they cause horses. Problems are usually limited to the horse's appearance as they will rub hair off their tails and some horses do become irritable.

Ascarids (roundworms) are more of a problem in foals and infestations can cause poor growth, rough hair coats, chronic respiratory problems, and sometimes death. The eggs are particularly hardy and will live for years in the environment, infecting foal crops from one year to the next. A few concepts were introduced before, namely that most of the damage caused by parasites is by their migration and that this damage is not always easily recognized as being caused by parasites. The lifecycle of the roundworm illustrates both these points clearly.

The foal eats the Ascarid egg (which may have been waiting on the ground for 8 years) and once in the foal’s stomach hatches into larvae and burrows into the intestinal lining and begins its migration. During its migration it enters the bloodstream and ends up in both the liver and the lung where it eats its way through the tissue. The liver regenerates and heals very well but this isn’t the case with the lung. Once lung tissue is injured it can only heal with scar tissue which is useless for breathing. This lung injury is the real damage of this worm; these injured lungs will never have the same air exchange capability as a normal lung and these foals are more susceptible to persistent respiratory problems such as pneumonia and shortness of breath throughout their lives. As the larvae develops it continues its destructive journey through the lungs and makes its way into the trachea. In the trachea it causes irritation and a cough which results in it being coughed up into the throat and swallowed. Once swallowed it enters the intestine a second time where it impairs the foals absorption of nutrients and eventually matures to the adult worm and lays eggs.

The less common Threadworms can cause diarrhea, skin irritation, respiratory infections and if in high numbers weakness and emaciation in the foal. These worms can be transmitted through the mares milk and it is therefore important to control the infection in the mare to protect the foal. Stomach worms can cause disfiguring skin lesions (summer sores), and Lungworms a persistent cough (usually associated with horses living with donkeys).

Large strongyles, the most common and dangerous intestinal parasite of the horse, is ingested by the horse in the larval stage. Once ingested the larvae migrate throughout the body and specifically to the cranial mesenteric artery, a crucial blood vessel at the base of the intestine’s circulation. Once there, they cause significant damage to the blood vessel which in turn compromises the circulation to the adjacent intestines. One result can be a burst artery and rapid hemorrhage and death. More commonly, the compromised intestine causes recurrent colics and all too frequently death (see below).

Lifecycle of Large Strongyles.

Eggs are produced by females in the intestines and are released onto pastures in manure. Eggs hatch into larvae and attach to grass where they live within drops of water or dew until they are eaten. In the horse larvae penetrate the intestines and migrate throughout the circulatory system. Different species of Large Strongyles attack different organs including blood vessels of the intestines and the liver. Larvae in the intestines mature to adults and lay eggs.

We can greatly decrease the incidence of these symptoms by controlling the severity of infection within horses. Because many symptoms are drastic it is always better to prevent intestinal parasites than to treat their effects.

Components of a Good Control Program

Parasite control is best achieved by a three fronted attack:

1. Decrease the number of adult parasites laying eggs.
2. Decrease the number of larvae and eggs eaten by the horse.
3. Evaluate the efficacy of the first two controls.

1) Decrease the number of adult parasites laying eggs.

In many stabling situations parasites will eventually find their way into the intestines of every horse. As previously described, our methods of housing horses favor and perpetuate high parasite loads by crowding horses together on pasture, limiting their "range" and allowing their food and water to be contaminated by egg and larvae infected manure.

The effective control program tries to decrease the number of mature adults, and in turn, the number of eggs shed. The first stage of parasite control, or killing the adults, is by regular deworming. This can be accomplished with a paste, "tubing" with stomach tube, or daily feeding of dewormer; all are good methods. This aspect of control is well known by most horse owners but the other aspects of control are often underused because they require more effort than deworming.

Any package of dewormer details the types of parasites it kills and therefore we will not detail the many products (except a few) available. There are many effective products and they fall under different chemical types. Many different brands exist for every type of dewormer and this must be remembered when considering rotation of dewormers. A general rule of thumb is to give the dewormers every 6-8 weeks, and in severe cases more often. A more precise interval can only be determined by fecal examinations mentioned below. Another relatively new product (Strongid C ¬) is fed on a daily basis and because of its success will be discussed in more detail below.

With the widespread use of dewormers we have seen an increase in the varieties of worms resistant to the effects of dewormers. This is a very important consideration in the choice of all dewormers.

In an effort to decrease parasite resistance many rotation strategies have been recommended. However, no consensus exists among parasitologists as to a superior rotation schedule. The strategies include changing the type of dewormer (not just brand name) at 1) each administration, 2) yearly, and 3) seasonally. An interesting study conducted in Tennessee several years ago compared different rotation plans and showed no difference between any of the strategies. Without knowing the specific parasite problems of a farm, it is always a guess to recommend any type of rotation scheme or even types of dewormer. Results are consistently improved if the choice and interval of dewormers are tailored for each farm. On some farms the rotation of Ivermectin, Strongid C ¬, and a Benzimidazole type dewormer has been successful. It is important to attack Bots from Spring until Fall and because of that we sometimes recommend Ivermectin: May to Sept., Strongid C ¬: Oct. - Jan. and Benzimidazole: Feb. - April.

The use of the popular dewormer Ivermectin (Eqvalan ¬, Zimectrin ¬) has truly had great results. Ivermectin, if used regularly, can actually resolve some, not all, of the damage Large strongyles cause in the intestinal blood vessels. Large strongyles are still as common as ever but it appears that the incidence of parasite induced colics have decreased. We think this is probably due to the extensive use of Ivermectin. Resistance to Ivermectin in equine parasites has not been seen, however resistance to Ivermectin has been seen in other animals. Also, Ivermectin does not kill all worms and with the routine use of this drug we have seen an increase in the number of tapeworms. Because of these reasons we recommend that Ivermectin not be used as the sole dewormer, but rather rotated with the other classes. We recommend Ivermectin be used from Spring until Fall so as to effectively treat for Bots which are commonly laying eggs during this time.

Daily dewormer (Strongid C ¬) is also used in many horses today. It’s use has shown some dramatic benefits because it doesn't allow the introduction of some worms at all. All other dewormers kill the worms within the horse only at the time they are given and can be thought of as "flushing" or purging the horse of parasites. As soon as the horse grazes it is immediately reinfected. In contrast, daily dewormers work by killing the worms as soon as they are introduced; the parasite doesn't have any chance to cause damage. Because of this it appears that Strongid C ¬ can improve feed utilization or put another way, the weight gained per pound of feed. It also seems to have helpful effects on some horses who are always thin and have a very difficult time gaining weight. One word of caution on its use is that we do not recommend it for foals and weanlings. When foals are exposed to intestinal parasites they develop some natural immunity to the parasites. There are reports that foals given daily dewormers never develop this immunity because the product does such a good job of killing these parasites. When these horses are sold as yearlings or older and are moved to other farms and not kept on daily dewormers they have serious reactions when finally exposed to intestinal parasites. We do not know of any scientific reports on this subject but nevertheless currently do not recommend its use on horses under 1 year. Because of its very valuable effects we do recommend its use on other horses and rotate it through a complete rotational cycle sometimes in the Fall through Winter. It is our experience the extra cost involved is well worth its effect and in many cases the expense is balanced by lower feed costs. It is always recommended that the day before this product is used Ivermectin be given to all horses.

Some special considerations need to be mentioned when discussing public stables. We have seen better parasite control in public stables that have taken control of the deworming procedure themselves or delegate it to a veterinarian. In public stables where owners deworm on their own the parasite problem is always more persistent and infections more abundant. This is because routine deworming is most effective if all horses are treated as a herd, namely at the same time with the same product. Despite the attempt of barns to coordinate their owners, it seems impossible to accomplish this with everyone deworming on their own. Because of these considerations we urge public barns which have shared paddocks or pasture to take the deworming responsibility away from the owner. This recommendation is often not a very popular one, but in every instance when this problem is treated on a herd basis rather than an individual one, we see better results.

2) Decrease the number of larvae and eggs eaten by the horse.

The second aspect of parasite control involves decreasing the number of larvae and eggs eaten by the horse. This is done by minimizing the fecal contamination of food and water. Unfortunately, the following methods have lost their appeal because of the extra effort involved and some of the basic misconceptions of parasites mentioned at the beginning of the article. An excellent way to decrease fecal contamination of food is to feed in bunkers, not on the ground. Clean stalls also decrease fecal contamination of food and water. Another method mentioned in the beginning of this article is cleaning of pastures. The pickup and disposal of pasture manure yields dramatic results which result in longer intervals between deworming and lower egg counts. In Kentucky and other areas of very high horse concentrations, pastures are cleaned 2-4 times monthly from early Spring until Fall. This can be done manually or with large vacuums built for the job. When manure is picked up the benefit of dewormers is increased because horses don't become immediately reinfected when turned out. Another effective yet easy method is to quarantine all new horses until they are dewormed. It is important not to dispose manure of quarantined animals onto the pasture as it defeats the purpose of quarantine. Other very valuable but often overlooked control measures include: composting or disposal of bedding (not on pasture), pasture rotation, avoiding pasture overstocking, and pasture care (clipping, harrowing). It should be noted that manure of horses eating Stongid C ¬ is not contaminated with the eggs of many parasites.

3) Evaluate the efficacy of the first two controls.

The third phase of parasite control involves monitoring the effectiveness of the other control procedures. Measuring the effect of all the control measures is critical in decreasing parasite numbers and damage. Because as was mentioned before, once the damage is seen it has been going on for a long time, and is often irreversible. The only method of measuring our success at parasite control is via microscopic evaluation of manure. A recommended protocol is to examine the manure of 20% of the herd twice yearly. Feces are examined just before a deworming and repeated in 1-2 weeks. Fecal exams can either look for the presence of eggs with a fecal flotation technique or measure the number of eggs per gram of feces with the McMaster technique. Both procedures are not necessarily required each time but in some instances using both will allow us to more accurately grade the severity of the parasite infestation. Routine monitoring of manure transforms deworming recommendations (time, product) from a guess to a diagnosis and allows the tailoring of the parasite control program to the individual farm. The greater number of horses living together, the more there is a need for this.

Although this article covered many ideas it truly was only the "tip of the iceberg" when discussing intestinal parasites in horses. If you want more information regarding intestinal parasites in horses contact your veterinarian or please feel free to give us a call.

Portions of this article were published in Northwest Rider magazine, May 15 & June 15, 1996.